1. Field of the Invention
The invention relates to a porous metal structure body suitably used for improving or adjusting characteristics of light metal alloy members such as an aluminum alloy member, and a method for manufacturing the same. In particular, the invention relates to a porous metal structure body for reinforcing bearings of internal combustion engine made by an aluminum alloy, which is enveloped in the bearing by casting and improves properties of the bearing, and a method for manufacturing the porous metal structure body.
2. Related Art Statement
Engines made of an aluminum alloy as one of light metal alloys has been widely employed in recent years for enhancing lightweight and for improving heat release ability of internal combustion engines. However, since the aluminum alloy is inferior to conventional cast iron in its mechanical strength, it has been a problem that the mechanical strength of members becomes insufficient by employing the aluminum alloy to a member exposed to high temperatures.
For example, a crankshaft of the engine is supported by a bearing with interposition of a bearing metal. The bearing comprises a bearing housing which is a member integrally molded with a cylinder block, and a crankshaft bolding member which is a member secured to the bearing housing with a plurality of fixing bolts. However, it is a problem that rigidity of the bearing becomes insufficient at locations immediately under a crankshaft journal where the bearing suffers a large pressure by explosion of a combustion gas, when all the bearings are made of the aluminum alloy. It is also a problem that noise and vibration are increased due to large clearance when the bearing is exposed to a high temperature, when all the bearings are made of the aluminum alloy. This is because, since the aluminum alloy has a larger thermal expansion coefficient than iron base materials, a large difference of the thermal expansion coefficient is caused between the bearing and crankshaft comprising the iron base material.
For solving such problems, Japanese Utility Model Application Laid-Open No. 63-150115 proposes a light metal alloy crankshaft supporting member of an internal combustion engine in which the inside of a member partitioned by the center line of a bolt hole for mounting a cylinder block and a curved crank journal supporting face is reinforced by complexing with a reinforcing fiber, which states that the volume ratio of the reinforcing fiber is preferably 20 to 40% in order to permit the thermal expansion coefficient to be approximately equal to the thermal expansion coefficient of the crankshaft.
Japanese Patent Application Laid-Open No. 60-219436 proposes an engine block in which the bearing of an aluminum alloy housing cap attached to the lower part of the main unit of an aluminum alloy cylinder block is formed by enveloping an iron base material by casting.
According to the technologies described in Japanese Utility Model Application Laid-Open No. 63-150115 and Japanese Patent Application Laid-Open No. 60-219436, the bearing is able to acquire an increase of mechanical strength that cannot be attained by the aluminum alloy only. The publications also state that rigidity of the bearing is largely improved and a proper clearance is maintained.
Japanese Patent Application Laid-Open No. 2000-337348 proposes a crankshaft bearing having a supporting structure for supporting the crankshaft of the internal combustion engine and a holding part for holding the supporting structure, wherein the material of the supporting structure is a porous material comprising a high silicon aluminum alloy having an approximately equal thermal expansion coefficient to that of the crankshaft material, and the material of the holding part is flowed into voids of the supporting structure.
Japanese Patent Application Laid-Open No. 2001-276961 describes a preliminary molded body of a porous metal comprising a base material of iron or iron base metal containing 10 to 40% by weight of chromium. The preliminary molded body of the porous metal is intended to form a metal composite member by a casting method in which a time lag is given between completing pouring of the molten alloy and impregnation of a molten alloy.
However, in the art disclosed in Japanese Utility Model Application Laid-Open No. 63-150115, it is a problem that the member which is the fiber reinforced composite, does not always have satisfactory characteristics under a high temperature environment. It is difficult by the art disclosed in Japanese Patent Application Laid-Open No. 60-219436 to select iron base materials for controlling the thermal expansion coefficient of the bearing to a desired value, and there is a limitation to decrease the thermal expansion coefficient. Furthermore, the bonding strength of the iron base materials to the aluminum alloy is insufficient.
Although the thermal expansion coefficient difference between the crankshaft and supporting structure is certainly decreased in the art described in Japanese Patent Application Laid-Open No. 2000-337348, the decrease is limited, and stable and satisfactory characteristics cannot be always obtained due to scattering strength at the boundary between the supporting portion and supporting structure.
Decrease of the thermal expansion coefficient is also limited in the composite member obtained by using the porous metal molded body and impregnating it with the aluminum alloy disclosed in Japanese Patent Application Laid-Open No. 2001-276961. The composite material may be exfoliated at the boundary due to scattering the strength of the boundary, thereby often failing in obtaining stable and satisfactory characteristics.
Since the porous metal molded body, or porous metal sintered body has a low strength, its handling is usually difficult. In particular, the porous metal molded body or porous metal sintered body having a low density is readily cracked to make it difficult to subject it to additional molding. In addition, a prescribed shape cannot be often obtained due to generation of exfoliation and cracks, when the porous metal molded body is formed into a composite material by, for example, being enveloping in a light metal alloy member by casting.